Water is a valuable resource. Some waters have a high sulfate content, such as acid mine drainage (AMD), which makes them unsuitable as water for many purposes, such as use in a hydraulic fracturing fluid, and may also make them difficult to dispose.
Acid mine drainage, acid and metalliferous drainage (AMD), or acid rock drainage (ARD), refers to the outflow of acidic water from metal mines or coal mines (usually abandoned). However, other areas where the earth has been disturbed (e.g. construction sites, subdivisions, transportation corridors, etc.) may also contribute acid rock drainage to the environment. In many places the liquid that drains from coal stocks, coal handling facilities, coal washeries, and even coal waste tips can be highly acidic, and in such cases it is treated as acid rock drainage. Acid rock drainage occurs naturally within some environments as part of the rock weathering process but may be exacerbated by large-scale earth disturbances characteristic of mining and other large construction activities, usually within rocks containing an abundance of sulfide minerals.
The same type of chemical reactions and processes may occur through the disturbance of acid sulfate soils formed under coastal or estuarine conditions after the last major sea level rise, and constitute a similar environmental hazard.
A significant amount of scientific research has been conducted to determine the chemical reactions that create acidity and lead to the precipitation of dissolved metals, but despite improvements in both prediction and prevention methods, acid mine drainage problems persist. The acidity of coal-mine drainage is caused primarily by the oxidation of the mineral pyrite (iron sulfide or FeS2), which is found in coal, coal overburden, and mine waste piles.
The sulfates contained in AMD or ARD may include, but are not necessarily limited to soluble sulfates, iron sulfate, sodium sulfate, calcium sulfate, magnesium sulfate, potassium sulfate, aluminum sulfate, zinc sulfate, and the like.
There have been many attempts to reclaim water, such as produced water from hydrocarbon recovery processes, and reuse it as fracturing feed water, commonly referred to as “frac water”. Frac water is a term that refers to water suitable for use in the creation of fracturing (frac) gels which are used in hydraulic fracturing operations. Frac gels may be created by combining frac water with a polymer, such as guar gum, and in some applications a cross-linker, typically borate-based, to form a fluid that gels upon hydration of the polymer. Several chemical additives generally will be added to the frac gel to form a treatment fluid specifically designed for the anticipated wellbore, reservoir and operating conditions. In many instances, it is not necessary to completely purify the water before it may be used as frac water. Frac gels may also be formulated by adding effective amounts of viscoelastic surfactants (VESs) to frac water.
It would thus be very desirable to discover relatively simple and inexpensive methods for reducing the levels of sulfates in waters containing them, particularly quickly and easily reducing the level of sulfates in water while not necessarily removing all of the sulfates or completely purifying the water.